1. Field of the Invention
The present invention relates to a method for manufacturing a minute silicon mechanical device, and more particularly to a method for manufacturing a minute silicon minute device which removes the diffusion region doped with an impurity of high density formed over a silicon substrate via an electropolishing process and thus limits the space where beams will be bent.
2. Discussion of Related Art
Recently, development has been made on sensors which can sense oil pressure, pressure, velocity, vibration, weight and the like using silicon. These sensors utilizing the excellent mechanical characteristics of silicon and the semiconductor manufacturing technology, permit the miniaturization, weight reduction and low price of the device. In addition, the peripheral circuits for signal processing of the sensors can be integrated on an identical chip and this enables enhanced reliability, sensitivity, signal to noise characteristics, and easy application to the system.
The sensor using silicon consists of a minute mechanical structure to which physical power, such as oil pressure, pressure, velocity, vibration, and weight is applied, a converting element for converting the physical power to an electric signal and a signal processor for producing a rated output.
The minute mechanical structure as a core part of the silicon sensor is manufactured using minute silicon processing technology and has a diaphragm, a cantilever, and an air-bridge.
FIGS. 1(A) to 1(C) illustrate the sequential steps of the manufacturing process of the minute silicon mechanical device according to the prior art.
Referring to FIG. 1(A), an N-type epitaxial layer 13 is formed over a P-type silicon substrate 11. The upper portion of the epitaxial layer 13 is thermal-oxidized to form an upper oxide layer 15 and the lower surface of the silicon substrate 11 is ground to a predetermined thickness via a back-grinding method. Thereafter, the lower surface of the silicon substrate 11 is thermal-oxidized to form a lower oxide layer 17.
Referring to FIG. 1(B), the edge portion including one of four edges of the lower oxide layer 17 and the center portion thereof are removed via a photolithography process, thus exposing the lower surface of the silicon substrate 11. In this case, the lower oxide layer 17 has the form of a small letter "n." By applying a backward electric field between the P-type silicon substrate 11 and the N-type epitaxial layer 13, the exposed lower surface of the silicon substrate 11 is etched with an anisotropic etchant, such as KOH, EPW (Ethylenediamine Pyrocatechol Water), Hydrazine or the like. In this case, the lower oxide layer 17 serves as an etch mask, and thus the silicon substrate 11 is etched to define three side walls 12. The silicon substrate 11 is anisotropically etched by this etchant. A depletion layer is formed at the boundary surface between the silicon substrate 11 and the epitaxial layer 13 by the backward voltage applied therebetween, thus preventing the epitaxial layer 13 from being etched. Hence, the epitaxial layer 13 becomes an etch stop layer and only the silicon substrate 11 is selectively etched. The silicon substrate 11 is normally etched at a temperature ranged from 60.degree. C. to 120.degree. C., however, the higher the temperature, the faster the etching speed.
Referring to FIG. 1(C), the upper oxide layer 15 is patterned via a photolithography process to be of the type that one end is connected and the other end, which does not face toward any of the three side walls 12 of the silicon substrate 11, is in stripe form. Then, the epitaxial layer 13 is patterned by using the upper oxide layer 15 as a mask, thus forming a plurality of beams 19 having a cantilever configuration. Thereafter, the upper and lower oxide layers 15 and 17 are removed.
In the minute silicon mechanical device manufactured as described above, the beams thereof are bent by physical power, such as external oil pressure, pressure, velocity, vibration, and weight, thus generating stress. This generated stress is converted into an electric signal at a converting element, thus sensing the strength of the power applied to the beams.
However, the manufacturing method of the minute silicon mechanical device as described above has some problems in that the silicon substrate is contaminated by the silicon particles generated while graining the lower surface of the silicon substrate and that the etching time is increased due to the anisotropic etching of the silicon substrate. Another problem is that the upper and lower portions of the epitaxial layer must be aligned to form the beams at the portion where the side walls are not formed, and this alignment is very difficult and may cause errors.